1. Field of the Present Invention
The present invention generally relates to the field of data storage systems and more particularly to data storage systems that uses multiple disks.
2. History of Related Art
Data-centric services are becoming increasingly common in the field of data processing networks. Data-centric services, as their name implies, are data storage intensive services such as file servers, web applications, transaction processing systems, search engines, and data repositories. Whereas reliability and performance of such services have traditionally received extensive attention because of the enormous commercial implications of these services and their corresponding systems, energy consumption has received increased attention more recently. The cost of power delivery, the cost of cooling system components, and the impact of high operating temperatures on the stability and reliability of the components have all contributed to emphasize the importance of energy consumption in the implementation of data centric services.
As stated in Gurumurthi et al., DRPM: Dynamic Speed Control for Power Management in Server Class Disks, Proceedings of the International Symposium on Computer Architecture (ISCA) pp. 169–179 (June 2003), recent studies indicate that a data center can consume several megawatts of power, that the power density of data centers could soon exceed 100 Watts per square foot, and that data centers could collectively require 40 terawatt-hours (TWh) in 2005 at an expected cost exceeding four billion USD.
A considerable portion of the power consumed by server class systems is attributable to the disk subsystem and a considerable portion of the energy consumed by the disk subsystem is attributable to the spindle motors. Consequently, efforts to reduce energy consumption in disk-intensive environments have focused at least some attention on techniques for reducing the amount of time the disk subsystem spindle motors are active. In early attempts, reduced energy consumption was achieved by operating each disk as a quasi-binary state. Each disk was either fully one (i.e., spinning at its maximum RPM to achieve the greatest performance) or fully off (i.e., not spinning) to achieve maximum energy conservation. In Gurumurthi, this approach was identified as limited and a variable RPM approach was suggested. According to Gurumurthi, individual disks could spin at one of multiple RPM values intermediate between an off state (i.e., 0 RPM) and some maximum RPM condition. In this manner, Gurumurthi suggests that better energy consumption can be achieved if one recognizes that the intervals between disk access events may not be sufficient to warrant a complete spin down and that a spin down of a particular disk may be warranted under only certain conditions.
While Gurumurthi suggests the benefit of employing a variable RPM disk scheme on individual disks within a disk array, it does not describe a mechanism for considering the individual disks, and their corresponding RRMs as part of an integrated whole. Gurumurthi describes a system in which an individual disk is responsible for determining its own RPM based on its loading, etc. In such a scheme for example, the RPM of one disk is wholly independent of the RPM of associated disks. It would desirable to implement a system in which the RPMs of various disks in a disk array subsystem were coordinated to balance the objectives of performance and energy consumption of the disk subsystem as a whole.